Will 1-2 Feet of 16 Gauge be a Problem for Solar?

[camper101]

I'm installing a panel on my roof and wondering if 1-2 feet (maximum -- ideally it'll be more like 8 inches) of 16 gauge wire will mess things up.

The reason I'd use it is to get the wire through a small hole and through the roof. I've got a side-entry cable clam that can accept wire up to 7mm. I really want the wire to be as flat and low as possible. Finding UV-resistant wire (with my experience anyway) has been tough, especially 2 tray wire.

To stay below the 7mm so I can get it through the hole, 16 gauge was the biggest wire available. The rest of the run is:

• 4 feet of 10 gauge from the panel (and will splice it to this 16 gauge section as close to the hole as possible)
• Then inside the camper they pre-wired with 12 gauge for the most part (that's maybe 20 feet of wire?)
• Then 3 feet of 10 gauge to the charge controller

The panel is a 90 watt Global Solar panel that might put out 5-6 amps -- under ideal conditions.

I've fiddled with DC/Wiring calculators, but I don't know what happens if you insert a short section of small wire in the middle.

So what would you say -- will I regret using that short section of small wire?

Thank you.

 

[oddvaro]

Your load current has to pass thru the entire cable. So use 16 gauge the entire length. No point in using thicker in any ends!


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[camper101]

Thanks Oddvaro,

The thicker wire on the ends is already in place – so the money has already been spent on copper by me or somebody else.

So I guess I'm wondering if small wire is like a “weakest link” problem (which it sounds like you're saying), or if it's more like a traffic problem: if you hit slower traffic for 5% of your total trip, the impact is minimal as long as you're cruising for the other 95% of the trip (and assuming the slowdown wasn't a major slowdown).

I guess the alternative to using the small wire is to use wire that isn't designed for a solar system, but I worry about what the sun will do to it over the years. I can try to cover it with split loom and electrical tape, but I'd like to have sturdy wire and zero maintenance if possible.

Thanks again

 

[DaveInDenver]

Your load current has to pass thru the entire cable. So use 16 gauge the entire length. No point in using thicker in any ends!


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In a sense yes, but wire resistance is ohms per length (usually specified at ohms per 1,000 feet or ohms per 1000 meters). So running a short length is better than a long length. In other words, the difference between 10 feet and 1 foot is an order of magnitude and not insignificant. The charts says 16 AWG is 4.016 ohms per 1,000 feet, thus 1 foot is about 4 mΩ (milliohms, e.g. 0.004 ohms) while 10 feet is 40 mΩ.

At 6 amps the difference is 240 mV (e.g. 0.24V) of drop compared to 24 mV. That's V = I x R, voltage is current times resistance, Ohm's Law. This is important because if you have 12.4V on one side of the wire, 10 feet down it'll be 12.16 but just one foot it'll be 12.376V. The smaller drop is essentially within the tolerance of your measurement the other can look like a fully charged battery compared to one that appears partially discharged.

You'll have to do the calculations for the whole circuit. Usually it's better not to drop down between wire gauges so much but a short length might be tolerable. The rule of thumb is to use 700 circular mils per amp of carrying capacity. Therefore a 6 amp circuit should be carried by 4200 circular mils, which is .0033 sq-inches or 0.06483 inches in diameter. That translates to roughly 14 AWG minimum.

But since you have relatively large wires in the rest of the circuit you might be fine. A length of 12 inches of 16 AWG at 6 A will experience 144 mW of heating (P = I² X R) or about 10°C temp rise by my back of napkin calc. This probably won't be hot enough to melt the insulation until you intend to be very close to the max for whatever insulation you're using. For example PVC is rated to 105°C, so you'd be hitting the high side for 1 foot at 95°C, which is just over 200°F.

Personally I would use 14 AWG if I could and if 16 AWG is absolutely required keep it as very short as possible and make sure you have solid connections on either end of it.

 

[camper101]

Thanks Dave.

So if I'm understanding, the thing I care about is the wire temp with zero current (sitting in the sun it might be hot) plus any temperature increase that comes as a result of the current. And I want that to stay (comfortably) below the rating of the wire. I haven't seen specifically what the wire is rated at, so I'm going with 90 degrees Celsius based on what I found online -- even better if it's 105.

 

[DaveInDenver]

Yes, you want the ambient temp + wire heating temp rise to below the max temperature rating for the insulation. You want a comfortable margin to be safe. The cross linked automotive wire types, GXL for example, are rated -50 to +125C, other types you need to research.

Ultimately all wire rating is about temperature. The insulation rating, the heating of the wire itself, etc. A fuse is nothing more than a length of wire that is designed to melt and a length of wire will eventually melt if allowed to heat enough.

In fact a fusible link is nothing but a short length of reduced diameter wire that melts and is contained within a special insulation that prevents the wire from melting through. So the question of reducing a circuit through a small gauge wire is really about sizing it primarily so it /doesn't/ become a fusible link.

So you do need to be very careful with what you are doing, you are at a very basic level considering making the pass-thru for practical purposes a fusible link. Something that popped into my mind was to put a bulkhead in and terminate both sides of larger wire with ring terminals. Or maybe drilling holes for proper cable glands.

 

[DaveInDenver]

 

[camper101]

Ok, so now I'm quite a bit more worried about heat than I was about voltage drop... as I probably should be.

So the alternative that I see other people using is basically something like 10 gauge wire that has (as far as I can tell) no protection against the sun. Most people seem to use these solar plugs with the wiring exposed.

If I go that route -- which would give me bigger wire -- does that mean I'll have to replace the wire more often (every few years, let's say) as the sun and road chemicals break it down?

Thanks again, learning a lot...

Edited to add: I have one of those solar plugs sitting around. I can steal a foot of wire from it and use that instead, but I figured I should use PV-specific wire if possible.

 

[4x4junkie]

14 AWG wire is rated to 15 amps in household circuits (where wires run next to wood wall studs & ceiling joists)... I would think a short piece of only slightly-smaller 16 AWG wire could handle at least 8-10 amps without heating up significantly. Your 5-6 amps is well under that.

My opinion: If there's no other way to do it, go ahead and use the piece of #16. Just keep it as short as possible, and fuse the circuit as appropriate for #16 wire.

FWIW, I've never had the sun break down the insulation on any of the common types of wires I've used in less than 10 years (though some of the lighter colors and reds I've occasionally had fade). This is the hot California desert sun too.

 

[camper101]

Thanks - good to know that your experience is the plain-old wire might hold up just fine. Guess I have to take my chances on one or the other: overheating, or wire that might need replacing someday.

 

[CaliMobber]

I first used speaker wire on my solar setup, the common black/red combo. After a year or so the plastic became very brittle and the plastic cracked so I replaced it. Ill have to see how long this new one last. I know what you mean about hard to find UV wire, its also very thick plastic and hard to work with.

I saw give it a try, if you realize it degraders fast or your not getting as much amps as you hoped, redo it another way.

 

[DaveInDenver]

14 AWG wire is rated to 15 amps in household circuits (where wires run next to wood wall studs & ceiling joists)... I would think a short piece of only slightly-smaller 16 AWG wire could handle at least 8-10 amps without heating up significantly. Your 5-6 amps is well under that.

Bear in mind that wire rating is about temp rise over ambient. Our houses rarely deviate from around 75F/25C as a starting point but a vehicle could experience air temps of 100F/37C and in some applications, such as chassis or engine wiring, might see quite higher. According to the SAE codes some wiring on your truck has to tolerate 125C/250F. Although in this case that wouldn't be necessary. At 125C the resistance of a length of wire is quite a bit higher than at room temp, about 50% higher. For this short of a length this isn't critical, a 16 AWG wire will be about 6 mΩ at 125C. But it's something to realize.

The main reason you need to be careful using generic ratings is that automotive wire (as in buildings) has a separate rating for the case when the wire is bundled. The same reasons that if you run commercial wiring in conduit you have to adjust your wire sizing because the whole bundle heats up differently due to the lack of air flow. The rating you normally see in charts assumes an unbundled wire. Paired you usually can assume the same, although technically you're supposed to derate even that. In a bundle of 15 (as you might find in a car harness) a conductor rated for 12 amps (e.g. 16 AWG) would be derated to 6 amps.

I do agree with you that 16AWG is /probably/ OK for a very short run at 6 amps but the rule of thumb on sizing fusible links is to go 4 wire gauges smaller than the wiring you're trying to protect. IOW, since his main harness is 10 AWG he's technically already there (16 AWG fusible links are for 12 AWG cabling).

Although in this case the 10 AWG is larger than necessary for the circuit, so the conditions are a little different. Those guides assume that a 10 (or 12) gauge wire is itself necessary to carry the current, so a 16 AWG wire shouldn't heat up enough to melt until you exceed the capacity of the larger wire. However in the future if someone (another owner, mechanic, who knows) sees the solar panels connected with 10 AWG wiring they might think the system is wired for more capacity and add more panels, which might start to tax the entry. I know it's conservative advice but a system should be consistently sized throughout.

 

[DaveInDenver]

Regards to the UV, I have cabling on my winch that was UV stabilized marine grade but it's bleached white now and starting to get brittle. I think it's from the heat being right in front of the radiator in an ARB bumper and exposed to the sun and road salts. I think ultimately we'll find shortcomings in UV tolerance or any other features of the wire, so we have to experiment to some extent. You'll find a wire and insulation that works but you'll have to add split loom or something to add the final piece. There's usually never one perfect solution for all the potential issues, so you have use some judgement to prioritize.

 

[Joe917]

Short answer: yes its a problem. Drill a bigger hole, find a larger clam. The 12 gauge wire is also adding to your voltage drop, undersized wiring is the biggest factor in solar under performance. With only a 90 watt panel you can't afford to waste voltage.
Read: http://handybobsolar.wordpress.com/the-rv-battery-charging-puzzle-2/

 

[Stumpalump]

I've had that problem where you just can't get a thick wire thru. I was able to fit two small wires and connected them in parallel. Two wires splitting the load is as good as one fat one.